FTR, there are a myriad of reasons why seats would be empty when they are sold.
They can be bought by people who want to sit together and therefore didn't select seats, they can be part of a travel agent or group booking where they're not assigned.
People can also skip the seat selection when booking their own tickets and won't be assigned until they checkin.
I was on a flight recently where the seat map was wide open an hour out and the flight ended up nearly full.
Seat maps can also show full when there are lots of seats blocked for bulkhead, pilots, or just weight and balance issues.
The bottom line is seat maps are cute but they prove zilch.
There's no connection between a seat map and predicting X, XN, I, IN, or R availability when J or Y is sold out.